1. Technical Field
The present invention relates to a liquid crystal display panel, a manufacturing method of the panel, and a liquid crystal display apparatus.
2. Background Art
A liquid crystal display (LCD) apparatus with many advantages such as thin, light weight, and low power consumption is used for many kinds of equipments such as Audio Visual equipments and Office Automation equipments.
FIG. 15 is a cross sectional view of a LCD apparatus. This LCD apparatus includes a LCD panel 32 and a backlight unit 33 as the main components. A LCD panel 32 includes a TFT (Thin Film transistor) substrate 34, an opposed substrate 36, a liquid crystal 35, and a sealant 38.
The TFT substrate 34 includes a plurality of switching elements such as TFTs being arranged in a matrix form. The opposed substrate 36 includes a color filter and a black matrix and the like. A liquid crystal 35 is arranged in a space between the TFT substrate 34 and the opposed substrate 36. Each of the TFT substrate 34 and the opposed substrate 36 is included with a polarizing plate 37. The sealant 38 is provided on a periphery of a LCD panel 32
As shown in FIG. 16, the TFT substrate 34 includes a scanning line 41, a data line 43, and a common line 42. A TFT includes a gate electrode, a drain electrode, and a source electrode. The gate electrode is connected to the scanning line 41, the drain electrode is connected to the data line 43, and the source electrode is connected to a pixel electrode 39. The common line 42 is a wiring for applying definite common voltage to a common electrode 40.
As shown in FIG. 16, the TFT substrate 34 has a display area 44 where TFTs are arranged in a matrix form, and has a terminal area 45 in a periphery of the display area 44. The terminal area 45 includes a terminal block 45a to work as a scanning line input terminal (first input terminal) for the scanning line 41, and a terminal block 45b to work as a data line input terminal (second input terminal) for the data line 43.
As a result, a scanning signal inputted from the first input terminal in the terminal block 45a performs an on/off control of this TFT, by entering into the gate electrode of the TFT flowing the scanning line 41. A data signal inputted from the second input terminal in the terminal block 45b flows the data line 43 and enters into the drain electrode of the TFT.
Then, when the TFT goes into an ON state, the data signal inputted to the drain electrode is applied to the pixel electrode 39 via the source electrode. As a result, an electric field arises between the pixel electrode 39 and the common electrode 40, and this field rotates liquid crystal molecules. Transmittance through the panel for the light from the backlight unit 33 changes, depending on the rotation angle of the liquid crystal molecules.
In FIG. 15, a symbol E denotes the electric field between the pixel electrode 39 and the common electrode 40. A heavy line arrow K1 indicates light from the backlight unit 33, and a heavy line arrow K2 indicates light which transmits the LCD panel 32.
Since a plurality of TFTs in pixels are connected to the scanning lines 41 and the data lines 43, the distance of each TFT from the terminal area 45 is different depending on the position of each pixel. A pixel at point A is illustrated by an example in FIG. 16 as a pixel in the position near the terminal area 45. And, a pixel at point B is illustrated by an example in FIG. 16 as a pixel in the position far from the terminal area 45.
Hereinafter, the pixel in the position near the terminal area 45 is described as point A, and the pixel in the position far from the terminal area 45 is described as point B. A scanning signal S1 inputted from the first input terminal in the terminal block 45a exhibits “delay”, “distortion of the signal waveform”, and “decline of the signal level” during flowing through the scanning line 41 because of the resistance of the line. In FIG. 16, a scanning signal S1a indicates the scanning signal S1 at point A, and a scanning signal S1b indicates the scanning signal S1 at point B.
Similarly, a data signal S2 inputted from the second input terminal in the terminal block 45b exhibits “delay”, “distortion of the signal waveform”, and “decline of the signal level” during flowing through the data line 43 because of the resistance of the line. In FIG. 16, a data signal S2a indicates the data signal S2 at point A, and a data signal S2b indicates the data signal S2 at point B.
In FIG. 17A, the vertical axis represents the value of the scanning signal S1a, the data signal S2a, and a common voltage S3 at point A, and the horizontal axis represents time. And in FIG. 17B, the vertical axis represents the scanning signal S1b, the data signal S2b and the common voltage S3 at point B, and the horizontal axis represents time. As shown in FIG. 17A, the waveforms of the scanning signal S1a and the data signal S2a at point A indicate almost the same rectangular shape as the waveform inputted to the first input terminal in the terminal block 45a. On the other hand, as shown in FIG. 17B, the waveforms of the scanning signal S1b and the data signal S2b at point B exhibit “delay”, “distortion of the signal waveform”, and “decline of the signal level” from the waveform, as shown in FIG. 17A, inputted to the second input terminal in the terminal block 45b. 
Meanwhile, symbols Va, Vb in FIGS. 17A and 17B denote the maximum voltage differences between the pixel electrode 39 and the common electrode 40, respectively, and reveals the magnitude relation of Va>Vb. Hereinafter, “delay”, “distortion of signal waveform”, and “decline of signal level” of the scanning signal and the data signal are collectively called as “signal degradation”.
FIG. 18 illustrates the characteristics of gate voltage versus drain current. Since the scanning line 41 is connected to the gate electrode of the TFT, fluctuation of the signal level of the scanning signal S1 causes fluctuation of the gate voltage. As the gate voltage fluctuates, the drain current shows different values.
The pixel electrode 39 and the common electrode 40 form a pair of electrodes of a capacitor. For this reason, a charged voltage and a charging time of the capacitor becomes different when the drain current flowing into the pixel electrode 39 changes. By the difference in such a charged voltage, liquid crystal molecules of different pixel are exposed to different electric field strength, thereby causing the difference in transmittance among pixels. Because the data line 43 has resistance similar to the scanning line 41, the drain current flowing into the TFT exhibits different values depending on the wiring resistance from the second input terminal in the terminal block 45b to the TFT.
Thus, by the delay and the like of the scanning signal S1 and the data signal S2, the transmittance differs among pixels. For this reason, the transmittance of the TFT substrate 34 shown in FIG. 16 partly declines, for example, in the order of upper left, lower left, upper right, and lower right. Thus the TFT substrate 34 exhibits the non-uniform transmittance distribution. Therefore, even if the backlight unit 33 illuminates with a uniform brightness over a surface of the TFT substrate 34, the LCD apparatus exhibits the display luminance distribution depending on the transmittance distribution.
To solve the above-mentioned problem, one possible method is to reduce the non-uniform distribution of the display luminance by controlling the brightness distribution of the backlight unit in accordance with the transmittance distribution.
For example, in a LCD apparatus with a backlight unit of an edge light system, a method to improve the uniformity of the display luminance distribution, by changing a reflector density in a light guide plate, is disclosed in Japanese Patent Application Laid-Open No. Hei 6-313883 (Document 1).
Another method to improve the uniformity of the display luminance distribution of a LCD apparatus, including a circuit to control brightness of a lamp, and a circuit to calculate a brightness correction factor for the lamp from a display data, is disclosed in Japanese Patent Application Laid-Open No. 2006-330187 (Document 2).
The TFT substrate 34 in FIG. 16 has a so-called one side extraction structure in which the terminal area 45 is arranged on one side around the LCD panel 32. On the other hand, the TFT substrate 34 may have a so-called double-side extraction structure in which the scanning line 41 and the data line 43 are divided into left and right sides, or top and bottom sides, and the terminal area 45 is arranged on the left and right sides, or on the top and bottom sides, respectively. In this structure, the distance from a terminal area 45 to the farthest TFT becomes shorter, thus the signal degradation by the wiring resistance becomes smaller.
In addition, non-uniform display luminance distribution of a LCD apparatus is also caused by non-uniform brightness distribution of a light source such as a backlight unit, besides the above mentioned existence of the wiring resistance of the scanning line 41 and the data line 43. Concerning this problem, Japanese Patent Application Laid-Open No. 2001-33782 (Document 3) proposes a method to control the transmittance distribution of a LCD apparatus so that the non-uniform brightness distribution of a flat light source can be compensated.
By the way, uniformity of the display luminance that is one of the image quality performances of a LCD apparatus is sometimes expressed in a specification by specifying luminance uniformity in full contrast display mode. However, concerning the image quality of the display apparatus for medical use, some specific standard values (or the request values) different from that for a conventional liquid crystal television are set, as shown in Table 1, like DIN (the industry standard of Germany: Deutsches Institut fur Normung e.V.), AAPM (the standard of the American medical physical society: American Association of Physicists in Medicine), and the like. Such regulations are requesting high uniformity of luminance in halftone display mode, in order to find out an affected part from an X-ray picture, for example.
TABLE 1DINAAPM - TG18EVALUATIONCENTER AND CORNERCENTER AND CORNERPOINTDISPLAYGRAY LEVEL: 50% OF MAXIMUM SIGNALGRAY LEVEL: 10% OF MAXIMUM SIGNALCONDITIONGRAY LEVEL: 80% OF MAXIMUM SIGNAL EVALUATION FUNCTION                                          (                                          CORNER                ⁢                                                                  ⁢                LUMINANCE                            -                                                                                      CENTER              ⁢                                                          ⁢              LUMINANCE                        )                                      CENTER      ⁢                          ⁢      LUMINANCE        ×  100  ⁢          ⁢      (    %    )                                            (                                          MAXIMUM                ⁢                                                                  ⁢                LUMINANCE                            -                                                                                      MINIMUM              ⁢                                                          ⁢              LUMINANCE                        )                                                                    (                                          MAXIMUM                ⁢                                                                  ⁢                LUMINANCE                            +                                                                                      MINIMUM              ⁢                                                          ⁢              LUMINANCE                        )                                ×  200  ⁢          ⁢      (    %    )   SPECIFICATION±15%±30%
Hence, for a LCD apparatus to be used also in the medical use, uniform luminance is required not only in full contrast display mode but also in halftone display mode. However, the difference in the transmittance caused by the above mentioned signal degradation becomes more serious in halftone display mode than in full contrast display mode. For this reason, the problem arises that luminance uniformity in halftone display mode becomes below the standard forms, even if luminance uniformity in full contrast display mode is within the standard.
FIG. 19 shows the voltage versus transmittance characteristics for a LCD panel. A full contrast display mode for a LCD panel is usually set in a voltage region R1 where the voltage-transmittance characteristics becomes flat. However, in such a setting, the voltage region for halftone display mode goes into the region R2 where voltage-transmittance characteristics changes steeply. Since the signal degradation affection appears more seriously in the region R2, the non-uniformity of display luminance distribution becomes larger in the halftone display mode.
FIG. 20A illustrates a surface of the TFT substrate 34 separated into a plurality of areas. FIG. 20B indicates the transmittance of each area in full contrast display mode, while FIG. 20C indicates the transmittance of each area in halftone display mode. As shown in FIG. 20B, when displayed in full contrast display mode the difference in the transmittance in each area is not so large. However, as shown in FIG. 20C, when displayed in halftone display mode, the transmittance for areas c and f away from the terminal area 45 tends to decline more substantially than the transmittance for areas a and d near the terminal area 45.
To this problem, the method, disclosed in the above-mentioned patent document 1, to adjust the light source brightness so that the luminance in halftone display mode becomes uniform can be applied. However, if the LCD adjusted in this way is operated by full contrast display mode, significant non-uniformity of luminance distribution will occur. This is because adjusting the light source brightness so that the luminance in halftone display mode becomes uniform is equivalent to making the brightness distribution of a light source non-uniform. Accordingly, when the LCD panel is operated in full contrast display mode, light from the light source with non-uniform brightness distribution transmits just as it is.
According to the method of the above-mentioned patent document 2, even when the transmittance distribution of the LCD panel is different between in full contrast display mode and in halftone display mode, the transmittance of the LCD panel can be adjusted by arbitrarily controlling distribution of brightness of the light source. However, in order to perform such control, a very complicated and expensive a light source controller is needed.
The problem that the luminance distribution is different between in halftone display mode and in full contrast display mode cannot be solved even in the double-side extraction structure of dividing the scanning line 41 and the data line 43, because wiring resistance does not vanish in any case.
In the above-mentioned patent document 3, methods of controlling the transmittance distribution of the LCD apparatus are disclosed, which include the methods to define a thickness of the liquid crystal layer, a ratio of light transmission area, and an electrode spacing of a comb-like shape electrode. However, since these methods can only be applied to the case of non-uniform brightness distribution of a light source, the methods cannot resolve the problem which is caused by wiring resistance of the scanning line 41 and the data line 43 and, thus, is independent of non-uniform distribution of brightness of a light source.